The present invention relates to the production of atactic or amorphous poly-alpha-olefins. More particularly it relates to a high temperature process for the production of non-crystalline or low crystallinity polymers and copolymers.
In the early processes for the production of isotactic or crystalline polypropylene, the initial catalyst systems (brown or violet titanium trichloride) were not very stereospecific and appreciable amounts of amorphous or atactic polypropylene were coproduced along with the desired isotactic polymer. These atactic or amorphous polymers were low melting non-crystalline or low crystallinity polymers with poor physical properties compared to isotactic polypropylene. The atactic or amorphous polymer had to be removed by solvent extraction so that the properties of the isotactic polymer were not degraded. At first, this material was burned or discarded by burial. Gradually, means were found to purify this material by removing the catalyst residues and solvent, and the resulting tacky, low molecular weight solid material found applications as an ingredient in hot melt adhesives, sealants, and bituminous formulations. However, the product quality was variable, with no two batches of material being exactly the same. The situation was further complicated by the fact that some material resulted from copolymer production and other material resulted from homopolymer production. The market for the amorphous material developed nicely. However, as the catalysts were improved, the amorphous polymer content was reduced to such a low value that it was not necessary to remove the atactic or amorphous polymer from the isotactic polypropylene. Producers found the demand for atactic or amorphous polymers outstripping the supply. Material which had been discarded earlier was mined from atactic pits and purified by acid washing and steam distillation. One polypropylene producer""s catalyst system for isotactic polymer was very poor and large quantities of atactic were produced, giving that producer a reasonable supply of amorphous poly-alpha-olefins (APAO). Another producer converted an isotactic polypropylene line to amorphous production and used a magnesium chloride (MgCl2) supported titanium catalyst minus the stereoregulator to produce xe2x80x9con purposexe2x80x9d APAO. This approach increased the quality and consistency of the amorphous product and allowed production of copolymers and terpolymers which had properties which were appreciably different from the coproduct atactic produced with isotactic polypropylene. This approach also opened up a potential for different materials which might have new applications.
U.S. Pat. No. 4,736,002 (issued Apr. 5, 1988 to G. Allen et al.) describes a process for preparing amorphous polypropylene and copolymers at relatively low temperatures of 150xc2x0 F. in liquid propylene, utilizing a converted isotactic polypropylene polymerization line. The disadvantages of this process are the necessity to operate the polymerization at low temperatures due to the inherent tackiness of the amorphous polymers and copolymers and the resulting low polymerization rates.
As a result of the building of new plants using the improved catalysts and the closing of old plants, there has occurred a shortage of atactic polypropylene.
It is an object of the present invention to provide a process to produce predominately atactic polypropylene for use in the roofing, adhesive and other markets. It is also an object of this invention is to provide a high temperature polymerization or copolymerization process in a tubular reactor to maximize the polymerization rate and reduce the stereoregularity of the polymer.
Another object of the present invention is to produce amorphous alpha-olefin polymers and copolymers which can be used in many applications. It is also an object of the present invention to prepare new polymers and copolymers which can not be produced by existing processes for preparing amorphous polyolefins.
The present invention provides a high temperature process for the production of amorphous polymers in the absence of solvent or a large excess of monomer used as diluent. As used herein, the term xe2x80x9cpolymerxe2x80x9d is intended to cover homopolymers, copolymers, terpolymers and the like, as well as graft polymers with polar monomer(s). The high temperatures employed in the process allow polymerization at very high rates and favor the formation of atactic polymer over isotactic polymer. Lack of stereoregularity is desirable for the end uses intended for these amorphous polymers. The polymerization is carried out in a tubular reactor at temperatures of 180-450xc2x0 F. in the presence of a catalyst system comprising a titanium chloride catalyst supported on magnesium chloride and activated with an aluminum alkyl without a stereoregulator. The catalyst system is selected to give amorphous polymers at efficiencies so high that the catalyst residues are insignificant and do not need to be removed. Excess monomer(s) are removed by devolatilization in two stages and catalyst residues are deactivated by the addition of steam. Antioxidants are added in the devolatilization section to prevent polymer breakdown during subsequent processing into hot melt adhesive formulations. In the tubular reactor, monomer may added at several injection points along the length of the reactor for purposes of cooling. The polymerization can be carried out in a stirred, pressure autoclave reactor as well. Pressures of 600 psig to 5000 psig can be used in the process depending on the monomer composition (higher pressures for higher % ethylene feeds) and the operability of the system at higher melt viscosities. Higher pressure will be used with higher viscosity materials, i.e., higher molecular weight materials for ease of operation of the reactor.
The amorphous polymer can be reactively coupled (i.e., grafted) with many polar monomers by adding peroxide and the polar monomer(s) after the devolatilization section in a melt pump or extruder.